I don't agree with the one FS thread/member/month rule. However, I agree that 20 threads for individual items is annoying, so I will propose this: A maximum of 5 current (ie, not closed, active) for sale threads per member. Personally I find throwing everything in one thread to be a bit unorganized. Most cases, it should be fine, but let's take this example.
I have one 280zxt, one scrapped 240z, and misc parts from misc years. For that, I would prefer to put JUST the ZXT parts in one thread, all the scrapped 240Z parts in one thread, and every thing else in a third thread. Much more organized and easier to deal with IMO. And it's not like 5 threads from one user will clutter up the FS section like 20 will.
I think this is a perfectly reasonable medium. My $.02.